Radioresistance correlates with expression of particular oncogenes in some cell systems, but not others. Further, since not all "radioresistant" cells express the same combination of oncogenes, the effects of oncogene expression may be modulated by cell-dependent factors. If there is a common radioresistant cell state, it is "downstream" from the initial expression of the oncogene(s). We propose to exploit two aspects of our recent work to attack this problem: 1) more detailed characterization of molecular events associated with human lung tumor progression; 2) more detailed characterization of radioresistance of human tumor cells. One of us has developed and characterized a series of human lung cancer cell lines, demonstrating the specific oncogene expression needed for transition from the more therapy-sensitive small cell lung cancer (SCLC) to the more therapy-resistant non-small cell lung cancer (NSCLC) phenotype. This transition has been characterized not only for changes in morphological phenotype, but also changes in relevant biochemical markers: TGF-alpha, EGF-R, PDGF and retinoic acid receptor gamma (RAR-gamma). Our major findings conclude that transition to the resistant phenotype can be effected by elevated expression of the mutated ras oncogene, complemented by elevated expression of c-myc or N-myc, not L-myc. Importantly, insertion of the RAR-gamma gene can also effect transition to the more resistant phenotype. Further, we have shown that SCLC cells transfected with the RAR-gamma gene express the sensitive or resistant phenotype depending upon extracellular levels of trans-retinoic acid (TRA). We will characterize radioresistance, both to acute and protracted radiation in these human lung cancer cell lines, and correlate radioresistance to oncogene expression as modulated by RAR-gamma. We have demonstrated that radioresistance to acutely delivered radiation is associated, in human tumor cells in general, with increased susceptibility to delay in G2-M phases of the cell cycle, where cells are rendered more sensitive to radiation. The proposed studies should determine whether any of the parameters characterizing radioresistance, including G2 delay, correlate with the molecular events associated with the change in phenotype from SCLC to NSCLC. If TRA inhibits transition to increased radioresistance, we will isolate genes that alter expression during such transition. Finally, we will transfect radiosensitive cells derived from human colorectal cancers with myc-ras and RAR-gamma vectors to determine whether these genes will increase radioresistance in other human cancer types. These studies should not only produce new insights into the relationship between expression of certain genes and radioresistance but may also suggest improved therapy regimens combining acute and protracted radiation, perhaps with TRA.